Diode-lasers are efficient devices for converting electrical power into coherent optical power. In this respect, they represent the most efficient class of laser devices. An edge-emitting diode-laser has a waveguide resonator that is typically hundreds of micrometers (μm) long and emits laser-radiation from an end facet. The laser-radiation from a diode-laser emitter is characterized as strongly divergent in a fast-axis direction and weakly divergent in a slow-axis direction. The fast-axis, slow-axis, and emission directions are mutually orthogonal.
For high-power applications, diode-laser bars having a plurality of diode-laser emitters in a “horizontal” linear array thereof provide a convenient way to scale optical power available from a single diode-laser emitter. For further power scaling, a plurality of diode-laser bars can be stacked together “vertically” to make a two-dimensional array of diode-laser emitters. Diode-laser bars arranged in this manner are typically referred to as a “vertical stack” or “fast-axis stack”. Diode-laser bars are preferably stacked with minimum bar-to-bar pitch, to maximize brightness of the laser-radiation emitted from the stack.
The term “packaging” applied to diode-laser bars refers to mounting a diode-laser bar on some sort of cooling-base or heat-sink. This base may be a relatively massive base providing a “conductively cooled package” (CCP). For higher power operation, the base is typically cooled by a liquid coolant that flows through a micro-channel arrangement. Micro-channels horizontally confine coolant flow within a cooler and typically have internal dimensions of less than 1 millimeter (mm). The coolant is usually water, or water with various additives.
A diode-laser bar includes a plurality of semiconductor layers epitaxially grown on a single-crystal substrate, with the plurality of diode-laser emitters defined in the epitaxial layers. Typically, the substrate is an n-type substrate, and layers are grown such that layers forming the “p-side” (anode-side) of the diodes are uppermost. The diode-laser bar is soldered “p-side down”, either directly onto the heat-sink or via a submount having a coefficient of thermal expansion (CTE) intermediate that of the substrate material and the heat-sink material, which is usually copper.
Electrical connection to the diode-laser bars places the heat-sink and cooling-water therein, in electrical contract with the diode-laser bar energizing circuit. The cooling water can short-circuit electric connection to the bars, unless the electrical conductivity of the water is low. A common solution to this is to use de-ionized (DI) or high-resistance water. However, DI water is more corrosive on metals than utility water from conventional building supplies, and the use of DI water is expensive and inconvenient by comparison.
Even small “stray” currents through the cooling water, between metal elements at different electric potentials, can cause metal corrosion through galvanic action. In addition to erosion of metal elements, particles produced by galvanic action can block cooling-channels in micro-channel cooled arrangements.
There is a need for an improved diode-laser bar assembly, having the cooling water electrically isolated from both the n-side and p-side electrical potentials. Preferably, DI water should not be required as the coolant. The diode-laser bar assembly should preferably be thin and stackable with a small bar-to-bar pitch.